In recent years, the market indicates a drastic increase of demand for wireless communication products, and thus the wireless local area network (WLAN) is developing in a much faster pace, particularly after the IEEE 802.11 WLAN protocol was established around 1997, not only providing unprecedented functions for the WLAN communications, but also providing a universal solution for various wireless products of different brands and creating a new milestone for the development of WLAN. The IEEE802.11 WLAN protocol confirms the total solution of executing a single chip by a core device, which greatly lowers the cost of adopting wireless technologies, so that wireless networks can be used extensively in various wireless communication products.
When these wireless communication products carry out the wireless data transmission under a high power condition, a high-frequency harmonic spurious noise is produced and an electromagnetic interference results. Therefore, every developed country has established related laws and regulations to solve this problem. The import and use of wireless communication products incompliant to the laws and regulations are prohibited. In view of such laws and regulations, wireless communication product designers and manufacturers add a wave filter behind a power amplifier for the circuit layout and design in a limited space to suppress the high-frequency harmonic spurious noises in hope of producing a wireless communication product in compliance with the related laws and regulations of different countries. However, when each manufacturer designs and manufactures the control circuit for such wireless communication products, the wave filter is usually used for suppressing the second harmonic waves produced by these wireless communication products, but the high-frequency filter not only comes with a high price, but also has a relatively large size. Therefore, adding a high-frequency wave filter will greatly increase the manufacturing cost and volume of the product, which is obviously not suitable for the present trend of the design of a light, thin, short, compact, low-price and good-quality wireless communication product.
Further, a harmonic wave produced by a high-power transmitted signal is usually the most difficult part to pass an electromagnetic interference test required for the wireless communication products. More particularly, the second or higher harmonic wave used as the primary frequency of the transmitted signal is the most difficult one to meet the standards. Therefore, designing a wireless communication product in compliance with related laws and regulations without greatly increasing the manufacturing cost and volume of the product becomes an important issue for designers and manufactures to overcome.